Best way to make space in your loft is to do a loft conversion.
You will need planning and building regs, also structural engineer .

Dependant upon the size and the monies you want to spend.
With certain lofts you can make space with small alterations to bearers and beams to allow for more space and still keep the roof supported. So there are quick wins.
But with a full loft conversion would be to create the dormer to give more head room and to get a structural engineer to look at strengthening existing joists to receive flooring panels to spread the weight.

hire an architect to produce drawings then a quote can be costed and get these passed.